Understanding testicular function and the process of spermatogenesis is limited by the lack of knowledge about local control mechanisms within the testis. The objective of this proposal is to address this gap in understanding by demonstrating that insulin-like growth factor (IGF) binding protein-3 (BP-3) is a critical autocrine/paracrine modulator of FSH actions in the testis. IGFBP-3 is a growth hormone dependent 38-58,000 kDa glycoprotein that binds with high affinity to IGF. Recently, it has been shown that BP-3 is the predominant IGF carrier protein produced by rat Sertoli cells in culture. FSH markedly lowers the concentrations of BP-3 in Sertoli cell conditioned medium. In contrast, IGF-I strikingly increases BP-3 concentrations. Since Sertoli cells in culture produce IGF-I and IGF-I amplifies the effects of gonadotropins on Sertoli cell function, the dual effects of FSH and IGF-I on BP-3 expression strongly suggest a physiological role for BP-3 in influencing the biological activity FSH and IGF-I in the testis. The principal hypothesis is that BP-3 is a principal mediator of local IGF activity and that its expression is regulated by FSH. The specific aims are 1) To determine whether BP-3 regulates the actions of FSH and IGF-I in the testis. The effects of purified BP-3 protein and antisense BP-3 oligonucleotides will tested on FSH and IGF-I induced androgen binding protein production and aromatase activity in cultured Sertoli cells. 2) To investigate the role of reproductive hormones in BP-3 regulation. BP-3 expression by RNAase protection assay will be measured during development, in hypophysectomized rats treated with FSH, and intact rats treated with high vs. low doses of testosterone. 3) To determine the sites of synthesis of BP-3 RNA and the location of BP-3 peptide. BP-3 will be localized by immunocytochemical and in situ hybridization techniques using representative tissue samples from the experiments in Specific Aim 2. These studies combining insights into the action, regulation and location of BP-3 should establish whether BP-3 is a physiological autocrine/paracrine modulator of IGF-I activity in the testis, and will have important implications for the control of male fertility.